Have the parental rights been terminated? Has there already been a custody determination by a court in favor of the grandparent? Based on the information you provided, I suspect that the religion angle may not the best thing to focus on.
If rights have not been terminated and this is an original proceeding, there is a presumption that it's in the best interest of child for the natural parent to have custody. The parent should only have to prove that she herself is fit, not that the grandparent is unfit because of her religion or any other reason.
But in addition to what has been provided, if the religion card is going to be played and the attorney is not familiar with these types of cases, I suggest reading and/or have the parents' attorney read Dealing with Jehovah's Witness Custody Cases by Jerry Bergman. Summary is here. The attorney will have the credentials needed to obtain the full article. Or if you PM me an e-mail address, I can send it to you.